In a hot-water supply system which includes a water softening device, there are various combinations of a hot-water supply device, a water softening device, and a regeneration device (specifically, a regeneration salt water supply device), and for example, an example of Patent Document 1 is as illustrated in FIG. 7.
In the hot-water supply system, a hot-water supply device 1 and a water softening device 2 are connected to each other by a data communication line 3, a remote controller 4 is connected to the data communication line 3, and the remote controller 4 controls the hot-water supply device and the water softening device.
The hot-water supply device 1 and the water softening device 2 are connected by a soft water supply pipe 5, and water, which passes through the water softening device 2 and is softened, is supplied to the hot-water supply device 1. The hot-water supply device 1 which is the example of the related art is a general device, and hot-water is supplied from a hot-water supply pipe 6. Moreover, in order to circulate the hot-water in a bathtub, the bathtub is connected by a return pipe 8a and a going pipe 8b. 
The water softening device 2 includes a water softener 11 of an ion exchange resin and a regeneration salt water supply device 12, and the regeneration salt water supply device 12 is connected to the water softener 11 via a salt water supply valve 13 and a salt water supply pipe 14.
Water softening capacity due to the ion exchange resin of the water softening device 2 is decreased according to an increased amount of water softening treatment. Moreover, in order to recover the water softening capacity, it is necessary to supply salt water from the regeneration salt water supply device 12 and perform regeneration treatment, and the regeneration treatment is performed at fixed intervals automatically.
Moreover, in the related art, in order to not interfere with the hot water when the regeneration treatment is performed, a bypass valve 7b is connected to the downstream side of a pressure reducing valve 7a of a water supply pipe 7, the water supply pipe 7 is connected to the soft water supply pipe 5 via the bypass valve 7b, and thus, hard water is supplied to the soft water supply pipe 5 by opening the bypass valve 7b. 
A water supply valve v1 is positioned between (at the upstream position of the water softener 11) a junction point of the water supply pipe 7 and the bypass valve 7b, and the hot-water supply device 1, a soft water supply valve v2 is positioned in the downstream position of the water softener 11, and a drain valve v3 is positioned in a drain pipe 9 of the water softener 11. Moreover, flow rate sensors S1 and S2 are positioned in the downstream position of the bypass valve 7b and the downstream position of the soft water supply valve v2 respectively.
During the regeneration treatment, the water supply valve v1 and the soft water supply valve v2 are closed, the water supply to the water softener 11 is interrupted, the salt water supply valve 13 and the drain valve v3 are opened, and salt water is supplied from the regeneration salt water supply device 12 to the water softener 11. The salt water which passes through the water softener 11 is discharged from the drain pipe 9 via the drain valve v3.
Moreover, one-time regeneration treatment ends within a predetermined time. A large amount of salt (for example, 20 kg) is replenished in the regeneration salt water supply device at once. However, if the regeneration treatment is repeated, since the salt is consumed and becomes deficient, it is necessary to appropriately replenish salt.
In addition, the regeneration treatment of the water softer 11 is appropriately performed, and thus, the water softening capacity is regenerated. However, the ion exchange resin is gradually deteriorated by repeating the regeneration treatment, and if the ion exchange resin is significantly deteriorated, it is necessary to exchange the ion exchange resin. Moreover, in a case of the ion exchange resin for home use, the service life is approximately 5 years.
However, if the regeneration treatment is not appropriately performed and the ion exchange resin is used in a state where the water softening capacity is decreased, the ion exchange resin is rapidly deteriorated, the regeneration capacity is decreased in a short time, and thus, it is necessary to exchange the ion exchange resin early.
On the other hand, the related art regarding the regeneration treatment of the similar water softening device is illustrated in FIG. 8 (Patent Document 2). In Patent Document 2, a resin tower 21 of a water softening device is disposed in a salt water tank 20, and a large amount of salt w is accommodated in the salt water tank 20. In the configuration of FIG. 8, water is supplied to the salt water tank, a predetermined amount of salt water Wa is generated, and the generated salt water is sucked up by a supply pipe 23, supplied to the resin tower 21 (water softener 11, the same as the water softening device), and regenerated.
In the configuration, an electrode Sw, which measures concentration of the salt water in the salt water tank 20, and a control unit are provided, and an alarm 22 which informs a timing in which the salt is to be replenished (the timing of salt replenishment), when such timing arrives, or the like is provided.
Water softening treatment and regeneration treatment are repeated automatically by switching a switching valve 23a. However, if the salt w in the salt water tank 20 is gradually consumed and becomes deficient, the concentration of the salt water after the regeneration treatment is suddenly decreased. Moreover, if the concentration of the salt water is less than or equal to a predetermined value, the alarm 22 is operated and informs that the replenishment of the salt w is needed.
The related art of Patent Document 2 is as described above. However, even when the alarm 22 informs that the salt replenishment is needed, the regeneration treatment is performed in a state where the salt is not replenished, and thereby, the water softening device may be used in a state where the ion exchange resin of the water softening device is not regenerated. Therefore, since the ion exchange resin is used in a state where the water softening capacity is decreased due to deficiency of the salt, the above-described problems occur due to early deterioration of the ion exchange resin.
Moreover, there is also an invention which extends a useful life of the ion exchange resin as long as possible by limiting the use of the water softening device to a necessary extent (Patent Document 3). In the related art of Patent Document 3, a bypass valve which bypasses a water softening device is provided, and for example, in a case where soft water is not needed, particularly such as hot-water for a kitchen, hot-water for a shower, bathroom heating, and hot water for bathtub, or the like, the bypass valve is opened, and thereby, the water softening device is not used.
As described above, in the hot-water supply systems which include the water softening devices of the related art, there is a problem in that the salt for the regeneration treatment becomes deficient, and the regeneration capacity of the ion exchange resin of the water softening device becomes rapidly decreased. Thereby, the water supply system is often used in a state where the water softening is not performed.